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Peter:

Just skip it for now --and maybe permanently. Skip the Sun-Sun index error test. You don't need that.

Could I ask what resource you're using to understand and set up these tests on your sextant? It matters because some resources get things wrong or with significantly misplaced emphasis. For example, many of the books available and nearly all of the instructional videos that you will find online will tell you to eliminate so-called "side error". Some of them are extreme about it and assert that "side error" must be aggressively expunged! But side "error" is no source of error at all. It's a misnomer. If there's a little side offset, it's nothing to worry about. Anything less than 5, 10, even 15 minutes of arc is no concern. Don't worry about it. Above that amount, side error can be a nuisance --a source of visual confusion. And it's possible (rare) to find a side "error" of several degrees, which should be resolved. Some sextant users actually prefer a little side offset. Makes it easier to do an index error test with a point object, like a star or planet. Some find that it's easier to determine when the direct and reflected images are exactly side by side (taking advantage of a little side offset) than to decide when they are perfectly superimposed (as they would be with zero side offset).

Frank Reed